Bias Linearity

The BIS.Net Team BIS.Net Team

Bias in the measurement system is the difference between the average of measured values and the actual value of a part. The actual value of the part is called the reference value. Ideally the bias should be zero.

Shewhart chart demonstrating the issues with spc theory

The above image shows the variation in the different measurements on the same part about the average and the deviation of the average from the reference value. In this instance the bias is negative.

A linearity study is performed to establish if there is a linear relationship with magnitude of the measured item. Usually, for a simple analysis several measurements are taken for each reference part by the same appraiser. The bias is then plotted on a scatter chart, as shown below, and a statistical analysis performed to determine if the observed linearity is likely to be real, or due to chance..

Shewhart chart demonstrating the issues with spc theory